A complete summary of Module 2 (Communication, Homeostasis and Energy) OCR A A-Level Biology, written by a current medical student who achieved an A* in all 4 A-Levels, including Biology. Condensed to 5 pages, easy to read and very clear layout. Written directly from the official textbook to ensure...
MODULE 5 - Communication, Homeostasis and Energy
5.1 - Communication and Homeostasis
● Cell signalling - neuronal or hormonal / endocrine system.
● Homeostasis = maintaining a constant internal environment, by responding to external
and internal factors / stimuli.
○ Achieved by negative feedback (conditions brought back to the optimum).
● Positive feedback = increases original change to destabilise the system (usually harmful).
○ Below a certain temperature, enzymes are less active and heat-releasing exergonic
reactions are slower so release less heat. Body cools faster, so enzyme activity further
decreases.
○ Cervix dilation - stretching of cervix causes increased secretion of oxytocin from
posterior pituitary gland, which increases uterine contractions and hence cervix
dilation. Birth ends oxytocin production.
● Ectotherms (rely on external heat sources):
○ Once active, muscle contractions generate heat from increased respiration.
○ Behavioural mechanisms, e.g. moving to/away from sun, or changing SA.
○ Advantages: less food used in respiration; more energy converted to growth; less
food needed; survive for longer periods without food.
○ Disadvantages: less active in cooler temperatures, so are more at risk from predators
and cannot take advantage of available food.
● Endotherms (metabolic reactions):
○ Advantages: maintain constant internal temperature; remain active in low
temperatures; inhabit colder environments.
○ Disadvantages: more food needed; may overheat in hot weather; use lower
proportion of energy for growth.
Organ Response when hot Response when cold
Skin ● Sweat glands secrete fluid ● Less sweat secreted.
onto the skin surface which ● Hairs stand erect to trap a layer
evaporates using body heat. of insulating air.
● Hairs lie flat to reduce ● Vasoconstriction of arterioles
insulation. and precapillary sphincters.
● Vasodilation of arterioles and
precapillary sphincters.
Gaseous Panting (increased evaporation of Less panting.
exchange water from lung surfaces).
system
Liver Less respiration to release less heat. More respiration in liver cells.
Skeletal muscles Fewer contractions to release less Spontaneous muscle contractions to
heat. release heat (shivering).
Blood vessels Dilation to direct blood to extremities Constriction to limit blood flow to
and lose more heat. extremities to avoid cooling.
● Thermoreceptors in the thermoregulatory centre of the hypothalamus detect blood / core
body temperature change, and transmit an action potential to counteract this.
● Peripheral thermoreceptors in skin monitor temperature of extremities, to inform the
hypothalamus and initiate behavioural mechanisms to prevent change to core body
temperature.
5.2 - Excretion as an Example of Homeostatic Control
● Excretion = removal of metabolic waste from the body.
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller daisyjackson1. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £4.99. You're not tied to anything after your purchase.
